The present invention relates to electronic circuits for use at a telephone subscriber's instrument, especially where it is necessary to provide a "features" telephone for telephone administrations that require a low voltage drop telephone. Many telephone administrations require the telephone subset to have a very low voltage drop across the line terminals when off hook (e.g. U.S. requirement of 6 volts at 20 mA)--to guarantee line "capture" at the telephone exchange. While it is possible to design a low voltage plain ordinary telephone (POT) the market requirement for more features telephones (which need more power) has been met either by offending the regulations, by using batteries/mains powering, or by using large inductors (e.g. 12 Henries, 100 mA rated).
Many POTs can provide 1 or 2 mA to external circuits such as signalling integrated circuits (ICs). However, the features telephone requires 2 or 3 mA for its microprocessor (if fitted) and as much power as possible (say 12-15 mA @ 3.5 volt minimum) for Call Progress Monitor (CPM) or Full Speech Monitor (FSM) or Handsfree (HF) operation to achieve satisfactory audio power output. Existing POT telephone ICs cannot meet these current demands.
FIG. 1 shows the configuration of the output stage of one such IC. In this, V.sub.L is the line voltage, I.sub.L is the line current, and I.sub.C is the current supplied to the telephone's electronic circuitry. Hence all the power for the internal circuits is drawn off from the line via the Z.sub.C network. This design restriction is caused by the modulation on the telephone line driving the instantaneous line voltage V.sub.L below the positive terminal of the power supply (which has to be derived from the line--as there is no other source of power). Also during transmission, power is wasted driving transmitter output current into the passive Z.sub.C network. In FIG. 1, V.sub.L =V.sub.REF+ (I.sub.C.Z.sub.C) where Z.sub.C is typically in the range of 600 to 1000 ohms, which limits I.sub.C to 1-2 mA maximum. R.sub.EXT is used to sense I.sub.L, for gain regulation, for example, and if I.sub.C is large then an error develops.
In the markets that allow it, it is possible to design a series-connected transmitter and loudspeaker power amplifier. The difficulty is voltage drop and even with the use of expensive VMOS polarity bridges in place of silicon diodes it is still not possible to meet many of the specifications on voltage drop.
Another difficulty with these features telephones is due to "on-hook" operation. Here the telephone is mechanically "on hook" but electrically "off hook" since the handset is in its cradle but the electronics fully or partially powered. Examples of this are the CPM/FSM and HF facilities referred to above. Traditionally this uses a complex mechanical interlocking switch, which although not very expensive is difficult to design and needs modifications for each telephone style. This design problem is eliminated with an electronic hookswitch but this has hitherto been rather expensive on components.